Make frog.py --leg work on Windows.

dart/lib/compiler/implementation/filenames.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#library('filenames');
+
+#import('dart:io');
+
+// TODO(ahe): This library should be replaced by a general
+// path-munging library.
+//
+// See also:
+// http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
+
+String nativeToUriPath(String filename) {
+  if (Platform.operatingSystem() != 'windows') return filename;
+  filename = filename.replaceAll('\\', '/');
+  if (filename.length > 2 && filename[1] == ':') {
+    filename = "/$filename";
+  }
+  return filename;
+}
+
+String uriPathToNative(String path) {
+  if (Platform.operatingSystem() != 'windows') return path;
+  if (path.length > 3 && path[0] == '/' && path[2] == ':') {
+    return path.substring(1);
+  } else {
+    return path;
+  }
+}